Many deposits of very fine-grained clay and kaolin clay known to be transformable to high-brightness clay by HIEMS (high-intensity electromagnetic separation), ozonation, and acid leaching cannot be utilized in commercial pigment production because of an inherent high viscosity and tendency to gel on ageing.
Previous attempts to reduce the viscosity of kaolin clays have utilized physical treatments such as viscous kneading (U.S. Pat. Nos. 2,535,647 and 2,907,666), high intensity agitation (U.S. Pat. No. 3,106,476) and pugging followed by treatment with hot water (U.S. Pat. No. 3,301,691).
Other approaches have combined physical and chemical treatments. Exemplary methods include pugging in an aqueous solution of urea (U.S. Pat. No. 3,510,330), pugging in an aqueous solution of an organic carboxylic acid followed by washing (U.S. Pat. No. 3,510,331), hydrothermal treatment or hydrothermal treatment with added monovalent and divalent cations (U.S. Pat. Nos. 3,614,075 and 3,765,825), extrusion followed by fractionation (U.S. Pat. No. 3,635,662), treatment with various acids and their salts including ascorbic acid, the pentasodium salt of diethylenetriamine pentacetic acid, citric acid, tartaric acid or gluconic acid (U.S. Pat. Nos. 3,892,587, 3,961,979, 4,144,083, 4,144,084 and 4,144,085), treatment with inorganic polymer (U.S. Pat. Nos. 4,030,941 and 4,105,466), reductive bleaching with an alkali metal hydrosulfite followed by treatment with barium ions (U.S. Pat. No. 4,182,785), dispersion in the presence of water soluble citrate and water soluble polyacrylate (U.S. Pat. No. 4,309,222), blunging with polyacrylic acid as a dispersant (U.S. Pat. No. 4,916,094), mechanical delamination (U.S. Pat. Nos. 5,169,443 and 5,411,587) and treatment with magnesium ions followed by pH adjustment (U.S. Pat. No. 5,707,912). All of these patents are hereby incorporated by reference in their entirety.
Despite the wealth of approaches to the preparation of reduced viscosity kaolin clays reflected in the aforereferenced patents, no one approach has been developed which is capable of consistently and reproducibly effecting a substantial reduction in viscosity of the initial clay.